The present invention relates to a method of deciding the risk of obstructive sleep apnea syndrome, a method of deciding sinking of the hyoid bone, programs, an X-ray diagnostic system and a method of making an oral appliance, which are preferably applied to decide the risk of a subject to become obstructive sleep apnea syndrome, or to decide the presence or absence or the degree of sinking of the hyoid bone, or to make an oral appliance for treatment of obstructive sleep apnea syndrome based on the result of decision.